Ultraviolet light produces death and mutation in simple organisms and skin cancer in man. Although cyclobutyl pyrimidine dimers in DNA have been implicated as a major cause of damage induced by UV in simple organisms, their role in UV-carcinogenesis in man is not yet clear. The photoreactivating enzyme, which repairs DNA by the exclusive monomerization of dimers in a light-dependent reaction is central to two phases of the problem of UV-damage, repair and carcinogenesis: (1) as a repair enzyme which may participate in DNA repair in normal cells and (2) as a specific analytical tool for evaluating the role of the dimer in the induction of abnormal growth by ultraviolet light. We shall study the mechanism of action of the enzyme by identifying the cofactor, determining the identity of the moiety responsible for the absorption of photoreactivating light and delineating the mechanism of dimer monomerization. We shall determine the amino acid composition, peptide maps and active site peptides of enzyme from E. coli, normal human fibroblasts and xeroderma pigmentosum fibroblasts. We shall determine the substrate range and specificity of the human photoreactivating enzyme. These studies will form the biochemical cornerstone of our determination of the role of dimers in carcinogenesis in man by ultraviolet light and of the role of the photoreactivating enzyme in repairing such damage.